Production of chemical wood pulps



Oct. '27, 1931. G. A. RICHTER 1,828,848

PRODUCTION OF CHEMICAL WOOD PULPS I Original Filed March 9. 1929/I/a,0// mam r Patented ()ct. 2'], 1931 GEQRGJE A. RICHTER, F BERLIN,NEW HAMPSHIRE, ASSIGNOR T0 BROWN COMPANY, 0F BERLIN, NEW HAMPSHIRE, ACORPORATION 0F MAINE rnonucrrou or cnniuroar. woon runes Applicationfiled March 9, 1929, Serial No. 345,869. Renewed March 27, 1931.

This invention relates to the production of chemical wood pulps bycooking in liquors of an alkalinetype, its object being the proatelevated temperature and under pressure in a liquor containing causticsoda and sodium sulphide in solution, both these chemicals reacting uponand dissolving the cementitious material which binds the Wood fiberstogether. is more drastic than sodium sulphide in its action oncellulose fiber, and accordingly that the higher the sulphidity of thecookina liquor. the better the quality of the resulting pulp. l haveobserved that degradation of the cellulose fiber o curs during the laststages of cooking, as at this time most of the cemeutitious materialpresent in the original wood has been dissolved. so that the causticsoda no longer reacts with a high;

degree of selectivity upon ,such material,

but reacts appreciably upon and degrades the fiber.

- In accordance with the process of the present invention, therefore,wood or other raw cellulo ic material is first cooked in a solution ofcaustic soda to effe t a removal of a portion of the cementitiousmaterial, but

without effecting fiber liberation. and is then cooked in'a solution ofsodium" sulphide to effect fiber liberatiomas it has been found thatwith this process the resulting pulp is of much betterthan usualquality: Such a process permits a removalof most of those cementitiousmaterials which are reacted upon and dissolved readily by caustic sodasolution, but without degrading the fiber, as

the caustic soda solution reacts with a high degree of sele tivity uponthe cementitious material of the wood before so much of it has beenremoved as to resultin fiber liberation. After the initial cooking incaustic soda solution, the wood isfin jcondition to be readily convertedinto-"pulp and when cooked in the milder sodium sulphide solu- It isknown that the caustic sodafor the recovery of their valuableconstituents, the spent caustic soda solution and the ill)- spent sodiumsulphide solution being mixed and their valuable inoraanic contentsmelted under reducing conditions to produce a smelt of sodium carbonateand sodium sulphide.

The sodium carbonate may be separated from the smelt as by selectivecrystallization. and then causticized for use in a subsequent firstcooking step, while the rest of the smelt may be sed in a subsequentsecond cooking step. The raw material from wh ch the chemicals areprepared for carrying out the process of the present invention may besimilar to those used in the kraift process, losses taking place in thecycle being made. up by adding sodium and sulphur constituent as inthe-form of sodium sulphate to the mixed spent liquors before deliveryinto the smeltinq furnace. onif desired. by addinfz caustic soda to thefresh caustic soda cooking liquor and sulphur to the mixedspent'liquors. In lieu of causticizing the smelted mixture; of sodiumcarbonate and sodium sulphide. however, the sodium carbonate may beseparated from the sodium sulphide as.

by selective crystallization, so that it may be lndependentlycausticized and the resulting caustic soda used in carrying out asubsequent independent initial cook 0? fresh,

Wood, whereupon the wood may then be cooked in a solution of sodiumsulphide-prepared from the remainder of the smelt.

The process of'the pre ent invention may best be understood from thefollowing moredelivered to an evaporator 4 with sufidcient caustic sodasolution .to maintain the wood immersed therein. The solution may varyin its alkalinity, containing, say, from about .5 to 5 pounds of causticsoda calculated as Na i) per cubic foot, weak solutions preferably beingemployed when cooking is to be collected under super-atmosphericpressure, and strong solutions preferably being employed when cooking isto be effected under atmospheric pressure conditions. The charge may becooked as by injecting steam directly thereinto until a substantialportion of the cementitious content of the wood has been reacted uponand dissolved, but, as previously stated, the concentration of alkali insolution, the temperature, pressure, and time employed in any cookshould be such as will not complete fiber liberation. After cooking hasbeen completed, the charge may be delivered into a washer 2, wherein thewood may be separated from the spent liquor withor without washing. Thespent liquor from the washer may be passed to a storage tank 3 fromwhich a portion may be withdrawn for use together with fresh causticsoda solution in subsequent cooking of fresh wood. The rest of thespentliquor may be subjected to treatment for the recovery of its valuableinorganic content, as will hereinafter be described. The wood from thewasher 2 may be delivered into a digester 4 of the type customarilyemployed for eflecting fiber liberation in alkaline liquors under heatand pressure. To this wood may then be added a solution of sodiumsulphide in amount sufficient to cover it. inasmuch as a substantialproportion of the resinous and ligneous matter present in t e originalwood has been removed, fiber li eration may be effected in a liquor oflower alkalinity than that usually employed whenthe usual onestcpcooking process is employed. lit desired, however, the solution may beof an alkali content similar to that of the usual kraft cooking liquors,in which case fiber liberation may be eli'ected in a shorter time thanordinarily. Using a sodium sulphide solution of an alkali concentrationof 2 to 2.5 pounds per cubic foot calculated as N220 and cooking at atemperature of about 300 to 335 F., it is possible to produce a highgrade pulp in a period of about three hoursl After fiber liberation hasbeen effected, the digester contents may be discharged into a washer 5,wherein a separation of pulp from spent liquor may be effected. Thespent liquor may be passed to a storage tank 6, from which a portion maybe withdrawn for use together with fresh sodium sulphide solution insubsequent cooking of other batches of wood coming from the washer 2.The rest of the spent liquor may be mixed with the excess spent causticsoda liquor accumulating in the storage tank 3, and the mixture servingto remove a portion of the aqueous content of the mixture and thus makepossible an economical recovery of its valuable inorganic content. Theconcentrated mixture may be fed into a furnace 8 maintained underreducing conditions, wherein the combustible content of the liquor maybeburned and its valuable inorganic content sm'elted to produce a smeltconsisting essentially of a mixture of sodium carbonate and sodiumsulphide. The hot smelt may be delivered into a body of water maintainedin a dissolving tank 9, the hot aqueous solution produced preferablybeing of a concentration permitting selective crystallization of sodiumcarbonate when cooled. Crystallization may be accelerated as by passingthe hot smelt solution through a cooler 10, the resulting sodiumcarbonate crystals being removed from the mother liquor in which themore soluble sodium sulphide remains in solution. The sodium carbonatecrystals associated with but little sodium sulphide may be dissolved inwater in a tank 11, to produce a sodium carbonate solution which maythen be causill ,ticized in a tank 12 to form-a caustic soda solution.If; desired, however, the sodium carbonate solution may be removed alongwith the desired amount of sodium sulphide,

then dissolved in water and causticized, so 935 that the first cook,which is carried short of fiber liberation, may be edected in a liquorcontaining much more caustic soda than sodium sulphide, and the secondcook, which is carried to complete fiber liberation, may be ml effectedin a liquor containing much more sodium sulphide than caustic soda. Thecausticized solution may be diluted in a tank 13 with water to form afresh liquor of alkali concentration desired for use in cooking freshWood in the digester 1. The mother liquor, containing sodium sulphideand but a rela tively small amount of sodium carbonate, may be dilutedin a tank 14 to the strength desired for use in the digester 4, itssodium carbonate content being causticized. if desired, before such use.Losses occurring in the cycle may be made up by adding sodium andsulphur constituent as in the form of sodium sulphate to theconcentrated mixture of spent liquors being delivered to the furnace 8.Or, if desired, sodium losses may be made up by adding fresh causticsoda to the-solution in the tank 13, and sulphur losses may be made upby adding sulphur to the mixture of spent liquors.

Une of the important advantages of the process herein described is thatit makes possible the production of strong and comparatively easilybleachable pulps. Experiments indicate that sodium sulphide'isparticularly effective as a delignifying agent. For instance, if rawcellulosic material, such as spruce wood chips, is cooked under theusual temperature, pressure, and time conditions til!) aeaaees untilfiber liberation is completed in two alkaline liquors of similar alkalicontent calculated as Na O, in one liquor all the alkali being presentin the form of caustic soda and in the other liquor say about 40% of thealkali being present in the form of'sodium sulphide and the rest in theform of caustic soda, it is found that the pulp produced in thesodiumsulphide-containing liquor is stronger and has a higher cookingstain (i. e., is more easily bleachable) than the pulp produced in theall-caustic soda liquor. In other words, sodium sulphide is apparentlymore eifective and selective in its action upon lignin than causticsoda. With this in mind, it is thus seen that if a portion only of thelignin and resin of raw wood is removed by cooking in a liquorcontaining largely caustic soda, a substantially all-sodium sulphideliquor then being used to cook the wood to complete fiber liberation,the resulting pulp will be stronger and more easily bleachable than whenfiber liberation is carried out to completion in an all-caustic sodaliquor or in a liquor containing caustic soda in excess of sodiumsulphide.

llhe example hereinbefore'given deals with the use of wood as the rawcellulosic material, but the process of the present invention may beapplied in the production of pulp from other'raw cellulosic materials,such as straw, corn stalks, bagasse, and the like.

Having thus described the process of the present invention and certainof its practical applications, it should be evident to those skilled inthe art that various changes and modifications might be made thereinWithout departing from the spirit or scope of invention as set forth inthe appended claims.

I claim:

' 1. A process which comprises cooking raw cellulosic material in asolution of "caustic soda short of effecting fiber liberation, and thencooking in a solution of sodium sulphide to complete fiber liberation.

2. A process which comprises cooking raw cellulosic material in asolution of caustic soda short of efiecting fiber liberation, thencooking in a solution of sodium sulphide to complete fiber liberation,mixing the spent liquors resulting from both cooking steps, andrecovering the inorganic content of the mixture.

3. A. process which comprises cooking raw cellulosic material short ofefiecting fiber liberation in a mixed caustic soda and sodium sulphidesolution in which the caustic soda preponderates in amount, and thencooking to complete fiber liberation in a mixed caustic soda and sodiumsulphide solution in which the sodium sulphide p'reponderates in amount.

l. A cyclic process which comprises cooking raw cellulosic material in asolution of caustic soda short of effecting fiber liberation, thencooking in a solution of sodium sulphide to complete fiber liberation,mixing the spent liquors resulting from both cooking steps, smelting theinorganic content of the mixture in a reducing atmosphere to producesodium carbonate and sodium sulphide,- separating sodiuin carbonate fromthe smelt,

causticizing the sodium carbonate for use in such subsequentfirst-mentioned cooking of wood, and using the rest of the smelt forsuch subsequent second-mentioned cooking.

cyclic process which comprises cook- 7 solving the smelt in water toform an aqueous smelt solution, selectively crystallizing sodiumcarbonate from the smelt solution, causticizing the sodium carbonate foruse for such subsequent first-mentioned cooking of wood, and using therest of the smelt solution for such subsequent second-mentioned cookingoi: wood.

I 6. A cyclic process which comprises cooking raw cellulosic material ina solution of caustic soda short of effecting fiber liberation, thencooking in a solution of caustic soda to complete fiber liberation,mixing the spent liquors resulting from both cooking steps, smelting theinorganic content of the mixture in a reducing atmosphere to producesodium carbonate and sodium sulphide, dissolving the smelt in water toform an aqueous smelt solution, selectively crystallizing sodiumcarbonate from the smelt solution, causticizing the sodium carbonate foruse in such subsequent first-mentioned cooking of wood, and causticizingthe rest of the smelt solution for such subsequent second-mentionedcooking of Wood.

in testimony whereof l have afixed my signature.

GEQRGE A. RICHTER.

Hil

